Pharmacology: Calcitriol is the active form of vitamin D3 (cholecalciferol). The natural of endogenous supply of vitamin D in man mainly depends on ultraviolet light for conversion of 7-dehydrocholesterol to vitamin D3 in the skin.
Vitamin D3, must be metabolically activated in the liver and the kidney before it is fully active on its target tissues. The initial transformation is catalyzed by a vitamin D3-25-hydroxylase enzyme present in liver, and the product of this reaction is 25-(OH)D3 (calcifediol). The latter undergoes hydroxylation in the mitochondria of kidney tissue, and this reaction is activated by the renal 25-hydroxyvitamin D3-1-α-hydroxylase to product 1,25-(OH)2D3 (calcitriol), the active form of vitamin D3.
The known sites of action of calcitriol are intestine, bone, kidney and parathyroid gland. Calcitriol is the most active known form of vitamin D3 in stimulating intestinal calcium transport. In acutely uremic rats, calcitriol has been shown to stimulate intestinal calcium absorption. In bone, calcitriol, in conjunction with parathyroid hormone, stimulates resorption of calcium: and in the kidney, calcitriol increases the tubular reabsorption of calcium. In-vitro and in-vivo studies have shown that calcitriol directly suppresses secretion and synthesis of PTH. A vitamin d-resistant state may exist in uremic patients because of the failure of the kidney to adequately convert precursors to the active compound, calcitriol.
Calcitriol when administered by bolus injection is rapidly available in the blood stream. Vitamin d metabolites are known to be transported in blood, bound to specific plasma proteins. The pharmacologic activity of an administered dose of calcitriol is about 3 to 5 days. Two metabolic pathways for calcitriol have been identified, conversion to 1,24,25-(OH)3D3 and to calcitroic acid.